1. Field of the Invention
The present invention relates generally to ceramic insulators, and more particularly to ceramic spark plug insulators and methods of making ceramic spark plug insulators.
2. Related Art
As illustrated in FIG. 1, conventional spark plugs 10 generally utilize a ceramic insulator 12 which is partially disposed within a metal shell 16 and extends axially above the metal shell toward a terminal end 18. A conductive terminal 20 is disposed within a central bore 22 at the terminal end 18. The conductive terminal 20 is part of a conductive center electrode assembly 24 disposed within the central bore 22. At the opposite or firing end 26, a center electrode 28 is disposed within the insulator 12 and has an exposed sparking surface 30 which together with ground electrode 32 disposed on the shell 16 defines a spark gap 34. Many different insulator 12 configurations are used to accommodate a wide variety of terminal configurations, electrode assembly configurations, shell configurations and the like. However, referring to FIGS. 1 and 2, the features of insulator 12 are representative of conventional contemporary spark plug insulators generally.
Insulator 12 is a monolithic ceramic article which is typically made by pressing a blank from spray-dried powder and subsequently grinding a near-net shape insulator preform (which allows for shrinkage) from the blank using a grinding wheel, and then firing the insulator preform to a high temperature sufficient to densify the preform and sinter the powder particles to form the finished insulator. Insulator 12 has a mast portion 36 that extends above shell 16 which is adapted to receive a spark plug boot (not shown) and which has a wall thickness sufficient to provide the necessary mechanical strength to the insulator, as it may experience stresses associated with handling and installation of the spark plug. Mast portion 36 houses a terminal stud 37 in the central bore 22 as shown, and in other configurations (not shown), may also house other portions of center electrode assembly 24. Insulator 12 also includes large shoulder 38 which is used in conjunction with turn-over 40 to retain insulator 12 within metal shell 16 during operation of an engine as pressure associated with the combustion gases presses outwardly against the insulator 12 and center electrode assembly 24. Insulator 12 also has a lower cylindrical portion 42 disposed in metal shell 16 proximate the threaded portion 43 of the shell.
Lower cylindrical portion 42 houses a three part (conductor/suppressor/conductor) glass fired in suppressor seal (FISS) 44 in the central bore 22 as shown, or in other configurations, another portion of center electrode assembly 24. Lower cylindrical portion 42 transitions through small shoulder 45 to a tapered core nose 46 disposed on a lower portion thereof. Small shoulder 45 is operative to engage shoulder 47 in shell 16, and together with large shoulder 38 and turn over 40 (or in other shell configurations (not shown) a preformed flange or shoulder) retains insulator 12 in shell 16. Tapered core nose 46 houses the center electrode 28 which may also include a sparking tip (not shown) as the sparking surface 30. Insulators 12 have a high dielectric strength, high mechanical strength, high thermal conductivity, and resistance to thermal shock sufficient for the high-temperature operating environment of an internal combustion engine.
Spark plug insulators used in internal combustion engines are subjected to high temperature environments in the region of about 1,000° C. In operation, ignition voltage pulses of up to about 40,000 volts are applied through the spark plug to the center electrode, thereby causing a spark to jump the gap between the center and ground electrodes. The purpose of the insulator is to ensure the integrity of the spark path and prevent the voltage pulses from finding other paths to ground, thereby diminishing the sparking performance of the plug. The high voltage and high temperature environment described can either degrade the performance of existing insulator materials or highlight performance limits associated with these materials. For example, the pressing processes leaves relics of the spray-dried powder which are known to have a detrimental effect on dielectric strength of the ceramic, since the cross-sectional area of the pressed blank is not uniform along its length in order to accommodate the shape of the insulator. Density gradients may be present so that some regions of the insulator are of lower density (higher porosity).
Referring again to FIG. 1, density gradients and regions of lower density frequently occur using the pressing methods described above at locations where the cross-sectional thickness of the insulator changes, such as either side of large shoulder region 38, or the region adjacent to small shoulder 45. These regions of reduced density have a lower dielectric strength, hence they are more susceptible to dielectric breakdown. As another example, the grinding processes used to form spark plug insulators remove a large amount of material from the pressed blanks. This material is typically reprocessed into subsequent batches of spray-dried powder, but is also a potential source of contamination. Such contamination can also introduce random, localized regions of reduced dielectric strength within the ceramic materials used for spark plug insulators. As another example, the grinding processes used to form the insulators also leave a relatively rough surface finish on the sintered insulator, which typically necessitates glazing of the terminal end or mast of the insulator, and promotes adhesion of deposits from the combustion process on the firing end.
Many different materials have been used or proposed for use in ceramic spark plug insulators, including various porcelains and metal oxides. Currently, the most commonly used materials are alumina-based ceramic materials, which also typically incorporate various glasses and other alloying constituents. Examples of alumina-based ceramic materials suitable for use as ceramic spark plug insulators include those described in U.S. Pat. No. 4,879,260 (Manning) and U.S. Pat. No. 7,169,723 (Walker). The ceramic materials used for the insulator are dielectric materials. Dielectric strength of a material is generally defined as the maximum electric field which can be applied to the material without causing breakdown or electrical puncture thereof. The dielectric strength of spark plug insulators is generally measured in volts per mil (V/mil). A typical value for spark plug RMS dielectric strength for a standard spark plug design used in many applications is on the order of about 400 V/mil at room temperature. Dielectric strength of the insulators used in spark plugs is also a function of temperature. High temperatures cause an increase in the mobility of certain ions allowing current to more easily leak through the ceramic. Any leakage of current leads to localized heating which gradually degrades the resistance of the material to dielectric puncture. It has been observed that resistance of insulators to dielectric breakdown also tends to decrease over the life of a spark plug due to thermal stress on the spark plug cycling under an applied electric field and due to attendant thermal-electrical fatigue thereof. The exact nature of the microstructural and/or compositional changes are not completely understood, but are believed to be associated with localized heating to temperatures sufficient to bring about partial melting of the ceramic material.
As manufacturers continue to increase the complexity and reduce the size of internal combustion engines, spark plug insulators are needed that have a smaller diameter. Currently, size reduction is constrained due to the required dielectric strength of the insulator over the service lifetime of the plug, which is directly related to the thickness required for the walls of the insulator. Another factor limiting size reduction is that more manufacturers are demanding a longer service lifetime from spark plugs such as requesting 100,000 mile, 150,000 mile, and 175,000 mile service lifetimes from spark plugs. The longer the desired service lifetime, the higher the required dielectric strength. Also, the higher the required voltage, the higher required dielectric strength. Previously to increase the service lifetime or dielectric strength of a spark plug the walls of the insulator were increased in thickness. However, the current demand for more compact spark plugs for modern engines prevents or limits the use of thicker walled insulators. Therefore, as engines shrink in size and as longer service lifetimes and higher voltages are needed in spark plugs, a spark plug having an insulator with an increased dielectric strength and a reduced wall thickness and size is needed.
Therefore, for a spark plug insulator of a given size and wall thicknesses, it would be desirable to increase the dielectric strength and thereby reduce the susceptibility to dielectric breakdown during extended periods of service at high voltages and high operating temperatures in order to promote enhanced spark plug, and thus engine, performance. Alternately, for a given performance requirement, it would be desirable to increase the dielectric strength of the insulator material and thereby promote reduction of the size and wall thicknesses of the insulator material, thereby reducing the space envelope associated with the spark plug and enabling use of this space for other purposes.